1. Industrial Field of the Invention
The present invention relates to a developer applied to an image-forming material and a method for forming images and more specifically to a developer effective for a negative-working image-forming material which can directly be converted into a printing plate by scanning the same with an infrared laser rays on the basis of digital signals outputted from, for instance, a computer or which can be applied to the so-called direct plate-making process as well as an image-forming method which makes use of the foregoing developer.
2. Description of the Related Art
Considerable progress has recently been made in the laser technology and, in particular, a high power, small-sized laser capable of emitting light rays having a wavelength falling within the range of from near infrared to infrared has easily commercially been available. These lasers are quite useful as light sources for recording used when a printing plate is directly prepared on the basis of digital signals outputted from, for instance, a computer (Computer to Plate: hereunder sometimes referred to as “CTP”). For instance, a solid state laser and a semiconductor laser emitting infrared rays whose wavelengths falls within the range of from 760 to 1200 nm are quite useful because of their high power as compared with other lasers emitting light rays having wavelengths beyond the foregoing range. For this reason, there has recently been an increased demand for an image-forming material quite sensitive to such a laser or an image-forming material whose solubility in a developer would greatly be changed through the irradiation thereof with the laser light rays.
There has been proposed, as such an image-forming material, those which make the most use of a radical addition-polymerization reaction. The material is in general imagewise exposed to laser light rays and then developed with an alkaline aqueous solution to thus give desired images. In this respect, there have been proposed two kinds of developing methods or a method comprising the steps of heat-treating the imagewise exposed material and then developing the same and a method comprising the step of developing the imagewise exposed material immediately after the imagewise exposure without carrying out any heat-treatment. The former suffers from a problem in that it requires the use of a large-sized installation and that the production cost increases since an automatic developing machine to be used must be equipped with an oven for heating. On the other hand, the latter likewise suffers from a problem in that the radical polymerization of the material through the exposure is not sufficiently accelerated, the exposed area is insufficient in the degree of curing and therefore, the printing durability of the resulting printing plate is low as compared with that observed for the printing plate prepared according to the former method. However, there has been desired for the development of an effective plate-making or developing method free of any heat-treatment since it is quite advantageous for users to omit the use of any oven for heating in the plate-making method.
In order to impart satisfactory printing durability to the resulting printing plate using a non-heating plate-making process, it is desirable that damages of image areas during the step for developing the same should be reduced as low as possible. However, it has been reported that the development of, for instance, a negative-working image-forming material comprising a photopolymerizable composition requires the use of a strongly alkaline aqueous solution containing, for instance, potassium silicate and having a pH value of higher than 12.5 (see the patent reference 1 listed below) and the use of a developer having such a high pH value arises a problem such that the image area is greatly damaged and the resulting printing plate has insufficient printing durability. Moreover, if such developing treatment is continued over a long period of time, problems arise such that the activity of the developer may undergo a variation in, for instance, the concentration of absorbed carbon dioxide due to changes of environmental conditions such as the carbon dioxide concentration in the air and the quality of the resulting printing plate may variously be changed due to the influence of such an increase in the carbon dioxide concentration and that the components of the plate-making material are accumulated in the developing bath to thus cause, for instance, clogging of pipe or duct.
To solve the foregoing problem or to control any reduction of the developing ability of a developer due to any change with time or aging and/or repeated use of the same, there has been proposed a method for developing a layer comprising a light-sensitive resin composition formed on the surface of a glass substrate with a mixed aqueous solution containing sodium carbonate and sodium hydrogen carbonate having a pH value of about 10 (see, for instance, the patent references 2 and 3 specified below). However, the inventors of this invention applied these developers to a lithographic printing plate precursor comprising an aluminum substrate provided thereon with a light-sensitive layer which comprises a thermally polymerizable light-sensitive resin composition and as a result, they found that the developers never showed any satisfactory ability of developing the non-image area thereof.
There has also been reported an alkaline developer to which an alkyl phenol type surfactant is added for solving the problem concerning the developing ability of a photoresist comprising a pigment dispersed therein (see the patent reference 4 listed below), but there have not yet been developed any developer having a sufficient developing ability and capable of providing a printing plate having sufficient printing durability.    Patent Reference 1: Japanese Un-Examined Patent Publication (hereunder referred to as “J.P. KOKAI”) Hei 8-108621;    Patent Reference 2: J.P. KOKAI Hei 5-88377;    Patent Reference 3: J.P. KOKAI Hei 11-65126    Patent Reference 4: J.P. KOKAI Hei 10-239858